This invention relates to the field of rfid (radio frequency identification) chip and antenna technology. More particularly, this invention relates to an rfid chip and antenna having an improved range of communication with an associated r.f. interrogation/reading device
A typical rfid chip and antenna are usually both incorporated into an ID tag, such as that shown and described in U.S. Pat. No. 6,154,137 issued Nov. 28, 2000, the disclosure of which is hereby incorporated by reference. Generally, an ID tag has the antenna and the rfid chip mounted on or encapsulated in a thin substrate, such as a polyethylene terephthalate (PET) substrate as disclosed in U.S. Pat. No. 6,373,708 B1 issued Apr. 16, 2002, the disclosure of which is hereby incorporated by reference. The antenna is usually a small loop antenna or a dipole antenna, and must be ohmically connected to the rfid chip. The usual loop antenna is a multi-turn planar ohmic conductor formed in any one of several known ways. One such technique is silver paste printing on a suitable substrate, such as the PET substrate noted above. Another known technique for forming a loop antenna is copper deposition on a substrate as practiced by RCD Technology Corporation of Bethlehem, Pa. The size of the coil (coil diameter and thickness) and the number of turns is determined by the requirements of a particular application, including constraints on the physical size of the ID tag. The function of the antenna is to provide electromagnetic transfer of information between the rfid chip and outside interrogation/reader devices, such as a host CPU, a user reading station, or the like; as well as to enable inductive transfer of electrical power from an outside device into the rfid chip to furnish electrical power to the active circuit elements within the rfid chip.
Many types of commercially available rfid chips are known at present, each having the standard internal functional components commonly found in an rfid integrated circuit. Such standard components include an RF and analog section, a CPU, a ROM and an EEPROM (see 1999 IEEE International Solid-State Circuits Conference publication 0-7803-5129-0/99, FIG. 9.1.1: RFID transponder IC block diagram). The rfid chip receives electrical power via the antenna when interrogated by an outside device, and communicates with the outside device using standard protocols, such as the ISO 14443 protocol or the ISO15693 protocol. Prior to installation of an ID tag on an object, information identifying the object to be attached is written into the ROM (read-only memory) incorporated into the rfid chip. Once this information is written once into the ROM, it cannot be written over or otherwise altered by any interrogation device. The rfid chip can be interrogated by an outside interrogation/reader device and can only supply the information to the outside device-i.e, it cannot alter the information stored in the ROM.
ID tags of the type described above having an rfid chip and an antenna are very useful for object tracking and are currently used in a wide variety of such applications. Many more applications of this technology are theoretically possible, but practical implementations have been limited in the past by size and cost constraints. These constraints have been recently addressed by improved semiconductor batch processing techniques to the extent that very small rfid chips and antennae can now be produced at a cost substantially less than the cost of the individual objects to which they are intended to be attached. For example, Hitachi, LTD. of Tokyo, Japan introduced the mu series rfid chip and antenna in 2004, with a chip size of 0.4 mm×0.4 mm and a cost at least one-third less than the price of rfid chips then on the market. Other semiconductor manufacturers have followed suit with their own competitive offerings.
FIG. 1 is a top plan view of a prior art ID tag 10 having an rfid chip 12 and a separate discrete antenna 14, both of which elements are mounted on a substrate 15. The rfid chip 12 is an integrated circuit containing the usual circuitry required for a functional rfid device, and is a separately fabricated device. These integrated circuit devices are typically manufactured using batch processing techniques which are well known to those skilled in the art. In general, multiple copies of the basic device design are built up on a large semiconductor wafer, after which the individual chips are separated from each other and combined with other discrete components.
For the FIG. 1 ID tag 10, the other discrete component is the antenna 14, which enables the rfid chip 12 circuitry to communicate with an outside interrogation device and also enables the electromagnetic transfer of energy into the rfid chip 12 to power the electronic circuitry contained therein. Since the useful operating range of an rf antenna is a direct function of coil area, antenna 14 is ideally a multi-turn coil subtending a much larger area than rfid chip 12 in order to provide as large an effective operating range as possible. Antenna 14 is typically either a separately formed discrete coil which is then adhered to substrate 15, or a metallic layer deposited directly on substrate 15 during formation of the coil.
The ID tag 10 of FIG. 1 is typically constructed by first fabricating the rfid chip 12 and antenna 14 as separate components, mounting components 12 and 14 to substrate 15, and electrically connecting antenna 14 to rfid chip 12. For this purpose, rfid chip 12 is fabricated with two ohmic connection pads 16, 17 to which the free ends 18, 19 of antenna 14 are bonded.
While the process of constructing ID tag 10 appears simple and staightforward, in practice the process is actually quite difficult to perform with a high degree of repeatable accuracy. This difficulty is primarily due to the small dimensions of the connection pads on the rfid chip; the requirement that the free ends 18, 19 of antenna 14 be precisely positioned over pads 16, 17 just prior to the bonding step of the process; and the additional requirement that a precise mechanical and ohmic bond must be made between the antenna ends and the connection pads. It is estimated that the cost of producing an ID tag of the type shown in FIG. 1 is: rfid chip 12: ⅓rd; antenna 14: ⅓rd; assembly: ⅓rd. As the physical size of the rfid chip is reduced, these difficulties in assembling a properly functioning ID tag, and the assembly cost, increase accordingly.
FIG. 2 illustsrates one recent approach made in the art to eliminate the difficulties in assembling an ID tag having separate rfid chip and antenna components. As seen in this FIG., an ID tag 20 is fabricated with an integrally formed rfid chip 22 and antenna 24 on a substrate 25. Because the antenna 24 is formed along with rfid chip 22 during the chip fabrication process, an ohmic connection is automatically created between the rfid chip 22 and the free ends of antenna 24. This “coil-on-a-chip” approach eliminates the costly bonding step and the difficulties associated therewith. bonding of discrete components in the ID tag assembly process, it introduces a severe limitation on the effective operating range of an ID tag fabricated according to this technique. Since the “coil-on-a-chip” ID tags are fabricated using integrated circuit batch processing techniques, the size of the antenna is extremely limited to the size of the dies produced. For example, the published operating range of one commercially available “coil-on-a-chip” ID tag is limited to a maximum distance of 3.0 mm. While this may be adequate for some specialized applications, such a small operating range is unsuitable for the majority of applications currently envisioned for ID tags.
One attempt to extend the operating range of a “coil-on-a-chip” ID tag is disclosed in U.S. Pat. No. 6,268,796 issued Jul. 31, 2001 for “Radio Frequency Identification Transponder Having Integrated Antenna”, the disclosure of which is hereby incorporated by reference. According to the teachings of this reference, an antenna is formed on a chip which is mounted above or below the rfid chip. The antenna has a number of coil turns which together constitute a helical coil whose axis is parallel to the major body plane of the rfid chip. To increase the inductance of the antenna coil, and thus the operating range of the ID tag, a high magnetic permeability layer is formed on the antenna chip. While this configuration does increase the operating range of a “coil-on-a-chip” ID tag, it requires several additional processing steps, which increase the fabrication cost and potentially affect the yield, and only provides an antenna with a relatively small area.
Thus, current “coil-on-a-chip” RFID tags still suffer from the severe disadvantage of a limited effective operating range with the associated interrogation/reader device.